1. Field of the Invention
The present invention relates to a thermoelectric material essentially formed of an Mg2Si-based compound, and to a method for producing the thermoelectric material. More particularly, the present invention relates to a thermoelectric material essentially formed of an Mg2Si-based compound synthesized from an Mg alloy containing Al, Zn, and Mn, which serve as alloy components and also as dopant elements; and to a method for producing the thermoelectric material, the method employing the liquid-solid phase reaction process in combination with sintering.
2. Background Art
Hitherto, there have been known thermoelectric conversion devices realizing conversion between thermal energy and electric energy. A thermoelectric module of such a device has a structure in which two types (P-type and N-type) of thermoelectric material (thermoelectric conversion materials) elements are connected in series electrically and arranged in parallel thermally. In such a thermoelectric module, when a difference in temperature is provided between two terminals, holes and electrons migrate respectively in the P-type and N-type materials on the basis of the Seebeck effect, whereby electromotive force is generated between the two terminals.
The figure of merit (Z:1/K) of a thermoelectric material is calculated by the following formula:Z=α2/(ρκ)(wherein ρ represents electrical resistivity (Ω·m), α represents Seebeck coefficient (V/K), and κ represents thermal conductivity (W/mK)).
Attempts have been made to utilize the aforementioned phenomenon in devices for power generators making use of waste heat discharged from various heat generating systems. Also, attempts have been made to utilize waste heat from automobile engines for power generation, since a considerably large amount of waste heat is discharged from automobile engines.
Hitherto, Bi2Te3, PbTe, or the like has been put into practice as a thermoelectric material for forming a thermoelectric conversion device. However, the element Bi, Te, or Pb, which forms such a thermoelectric material, has high toxicity, and thus may cause environmental pollution. Therefore, demand has arisen for a thermoelectric material having reduced environmental burden (i.e., a thermoelectric material having no toxicity). Also, demand has arisen for a lightweight, resourceful thermoelectric material, from the viewpoint that the material is used for recovery of waste heat from automobiles.
Mg2Si (specific weight: about 2) has been known as a nontoxic, lightweight N-type thermoelectric material for use under mid to high temperature. For improvement of its thermoelectric performance, Mg2Si is doped with a dopant. Known dopants added to Mg2Si include an element which provides Mg2Si with N-type semiconductor properties, such as Al or Sb, and an element which provides Mg2Si with P-type semiconductor properties, such as Ag or Cu. However, since Mg is an active metal and may cause the risk of, for example, ignition, Mg2Si thermoelectric materials have been less developed.
One method for producing such an Mg2Si-based compound is based on the direct melting process. Specifically, a powder mixture of Mg powder and Si powder (atomic ratio of Mg:Si=2:1) and powder of a dopant element, or a powder mixture of preliminarily produced Mg2Si powder and powder of a dopant element is heated to a temperature equal to or higher than the melting point of Mg2Si (1,085° C.), and subsequently an Mg2Si-based compound is produced under cooling. However, in this production method, a high-pressure inert gas must be loaded for suppressing evaporation of Mg, high cost is required for, for example, apparatuses or production steps, and the risk of explosion of Mg may be involved during heating. In addition, segregation of Mg or Si is likely to occur, and thus the resultant product may have a non-uniform composition (i.e., a difference in shrinkage between a center portion of the product and a portion thereof at the vicinity of a crucible), resulting in occurrence of cracking in the product.
In another method for producing an Mg2Si-based compound, a powder mixture of Mg powder and Si powder (atomic ratio of Mg:Si=2:1) and powder of a dopant element, or a powder mixture of preliminarily produced Mg2Si powder and powder of a dopant element is placed into a carbon crucible contained in a pressurized container filled with an inert gas, and the powder mixture is melted through high-frequency heating. However, similar to the case of the method employing the direct melting process, cracking may occur in a product produced through the method involving high-frequency heating. In addition, the method involving high-frequency heating possesses a problem in that high cost is required for producing a product of interest, since an expensive heating apparatus is employed, and the method per se is not suitable for large-scale production.
Yet another method for producing an Mg2Si-based compound employs the mechanical alloying process. In this method, Mg powder and Si powder are weighed so that the atomic ratio of Mg to Si is adjusted to 2:1, and these powders are milled by means of iron or ceramic balls for a long period of time (e.g., 300 h), to thereby mechanically synthesize Mg2Si powder. The thus-synthesized Mg2Si powder is mixed with a dopant element, and the mixture is thermally treated, to thereby produce an Mg2Si-based compound. In the production method employing the mechanical alloying process, conceivably, balls may be ground through long-term mixing and milling, and the resultant alloy powder may be contaminated with components of the balls (i.e., impurities), resulting in low purity of the powder. In addition, there must be taken into consideration the risk of explosion of Mg powder, which would otherwise occur when the powder is removed from a pot.
Still another method for producing an Mg2Si-based compound employs the discharge plasma process. In this method, Mg powder and Si powder (atomic ratio of Mg to Si=2:1) are mixed with powder of a dopant element; the resultant powder mixture is heated and retained at a temperature of 650° C. (melting point of Mg) to 800° C. for a specific period of time, to thereby form Mg2Si through reaction of molten Mg and Si particles, while the dopant element is dissolved in molten Mg so as to form a solid solution through substitution of the dopant element for a portion of Mg or Si contained in an Mg2Si crystal structure, to thereby produce an Mg2Si-based compound having a non-equilibrium composition.
As a technique relating to this production method, for example, Japanese Patent Application Laid-Open (kokai) No. 2002-285274 proposes an invention relating to “Mg—Si-based thermoelectric material and production method therefor”. This patent document discloses that an Mg—Si-based thermoelectric material is produced through a method including a step of mixing Mg powder and Si powder (atomic ratio of Mg to Si=2:1) with powder of a dopant element; a step of heating the powder mixture obtained through the mixing step at a temperature of Tm (melting point of Mg) to 1073 K for a specific period of time, to thereby form Mg2Si through reaction of molten Mg and Si particles, while dissolving the dopant element in molten Mg so as to form a solid solution through substitution of the dopant element for a portion of Mg or Si contained in the aforementioned Mg2Si crystal structure, to thereby produce an Mg2Si-based compound; and a cooling step for stopping the heating step after the elapse of the aforementioned specific period of time so that unreacted Si particles remain.
Meanwhile, Japanese Patent Application Laid-Open (kokai) No. 2006-128235 discloses that an Mg2Si-based compound is melted and doped with two dopant elements (Al and Zn) in an inert gas, to thereby synthesize an N-type thermoelectric semiconductor; and the semiconductor is crushed and then sintered by spark plasma sintering. The invention disclosed in this patent document overcomes problems in that doping of only Al causes considerable segregation, and doping of only Zn forms a P-type thermoelectric semiconductor, but fails to form an N-type thermoelectric semiconductor. This patent document describes that the total amount of the dopant elements Al and Zn is 0.21 at % to 2 at %.
The method disclosed in Japanese Patent Application Laid-Open (kokai) No. 2002-285274 can produce products of uniform quality and keep safety in manufacturing, and thus is useful for improvement of the thermoelectric performance of a product of interest and for reducing production cost, as compared with any of the aforementioned other production methods. However, there is still a question regarding the necessity of the cooling step for stopping heating so that unreacted Si particles remain.
Japanese Patent Application Laid-Open (kokai) No. 2002-285274 also discloses an Mg2Si-based compound represented by the chemical formula Mg66.667-xSi33.333-yAx+y (wherein A represents any dopant element selected from among Al, P, Ga, As, In, Sb, Ag, Cu, Au, Ni, Fe, Mn, Co, Zn, and Pb; and 0.017≦x≦0.192 and y=0, or x=0 and 0.017≦y≦0.192). However, in the Examples of this patent document, the material used for forming the compound is limited only to MgAlSi, and materials other than MgAlSi are not described. Thus, Japanese Patent Application Laid-Open (kokai) No. 2002-285274 does not suggest a thermoelectric material formed of Mg2-x-y-zAlxZnyMnzSi (i.e., an Mg2Si-based compound doped with a specific combination of the three elements Al, Zn, and Mn). The Mg2Si-based compound disclosed in this patent document is doped with only a single dopant element (0.017 at % to 0.192 at %).
The method disclosed in Japanese Patent Application Laid-Open (kokai) No. 2006-128235 realizes production of Mg2Si devices of uniform quality with reduced segregation. However, this method still fails to overcome a problem in terms of the performance deterioration of an Mg2Si device in air at high temperature. In addition, Japanese Patent Application Laid-Open (kokai) No. 2006-128235 discloses only an MgAlZnSi compound wherein the total amount of Al and Zn added is 0.21 at % to 2 at %, and does not suggest a thermoelectric material formed of Mg2-x-y-zAlxZnyMnzSi.